You know that feeling you get when you realize you’ve left the car unlocked overnight? That may be what American Airlines felt like after two passengers were able to walk onto an apparently unlocked plane sitting at Philadelphia International Airport without showing anyone their boarding passes.

American Airlines is calling it a case of a case of “unauthorized access,” WCVB.com reports: a man and his 21-year-old daughter simply walked onto the plane stowed their luggage, then went back out to the gate area to wait for the flight to Boston.

“Someone went through the door first,” the father explained to WCVB. “We followed him.”

Another passenger told the station that the captain of the flight said the plane was open all night.

“The pilot said the jetway door was open overnight,” he said. “The plane was not secure overnight,” the pilot added.

The flight was delayed three hours as the airline swapped planesjust to be on the safe side.

“Two passengers gained unauthorized access to one of our aircrafts earlier this morning,” American Airlines said in a statement. “Out of an abundance of caution, we are utilizing a different aircraft for the flight to Boston. We apologize to our customers for the inconvenience.”

Investigators are now looking into how the passengers were able to open a secured door, but foul play is not suspected.